Previously known is a so-called electret field, i.e. a permanent electric charge injected into a dielectric material by ionizing. A film applicable for use as a sensor film in the sensor element of the invention is presented in U.S. Pat. No. 4,654,546, in which a dielectric plastic film, such as polypropylene, containing flat or torn gas bubbles or voids is used to form a so-called cellular electret film. Both surfaces of the film are metal-coated. WO specification 96/06718 presents a method for swelling a foamed plastic film, whereby the amount of gas contained in the film can be more than doubled. Patent specification FI 913741 presents various electric structures for sensor elements. Previously known are also fibrous polarized electret films, as presented e.g. in U.S. Pat. No. 4,874,659. Other known elements applicable in the sensor element of the invention are piezoelectric sensor films, such as PVDF.
Sensor elements and sensor bands (ribbons) as provided by the invention, which have a relatively large area or length, connected to a suitable signal processing apparatus or system, can be used for many different purposes. Possible applications are for example sensors installed in a road structure for determining the weight of a moving vehicle, registration and monitoring of a patient's vital functions (breathing, heartbeat and snoring) using a sensor placed in the bed under the mattress e.g. in conjunction with sleep research, monitoring of the vital functions of a drunken person by means of a sensor installed in the floor of a jail, sensors mounted under a carpet in an old-age home to monitor an old person's getting up from bed, sensors mounted under a floor coating in the vestibule of a bank or shopping center and connected to an alarm system. From a long sensor band installed on a fence around an industrial area, using suitable software, it is possible to notify a possible intruder and its location when both ends of the sensor band are connected to a signal processing device. A sensor element may also be mounted inside a floor structure under a large machine, such as a paper machine, to monitor its operation. They can also be used in various safety applications, e.g. to make sure that a machine will not be started before its operator is in the proper place, and so on. In addition, this type of sensor elements can be used in sports e.g. to measure the force and duration of exertion. They can also be used as different kinds of switches, such as light switches, or in underwater ultrasound measurements. Moreover, a signal, e.g. an ultrasound signal can be fed into the sensor to make it vibrate. Sensors according to the invention can also be used in various keyboards, in cages for test animals for the measurement of motional activity, monitoring of the vital functions of an animal recovering from a surgical operation, and so on. When such large sensor elements are applied, a plurality of sensors are used and the signals obtained from them can be compared and summed to eliminate unnecessary signals or to draw other conclusions, e.g. by installing several sensors of opposite polarities in the floor of the same space it is possible to eliminate e.g. signals produced by the vibration of an air conditioner or the building itself and pick out the breath and pulse of a person lying on the floor. Traditionally, this type of large sensor elements or bands have been manufactured by cutting a sensor element of a size suited for the intended use e.g. from sensor material coiled up on a roll and consisting of a metal film with sensor films, e.g. electrically charged electret bubble films, laminated on its both surfaces so that the positively charged sides lie against the metal film, with further metal films laminated against the negative side on the outer surfaces of the laminate thus formed. When a force is applied to such an element, an electric charge is generated between the signal electrode in the core and the earth electrodes on the outer surfaces. A metal electrode may also be placed directly on the surface of the sensor film, e.g. by evaporating, as described e.g. in U.S. Pat. No. 4,654,546. Another commonly used method is to print an electrode pattern of silver pasta on the surface of a polyester film and laminate it together with a sensor film. With the first-mentioned methods, a problem in the manufacture of large sensor elements like this is that the sensors are sensitive to electromagnetic interference and discharges of static electricity. This is due to the fact that, as the sensor is cut from material in which the signal electrode is of the same size with the sensor material, it extends to the very edges of the sensor element. Therefore, the edge areas of the elements need to be separately provided with metal films extending over the edges, these metal films being grounded. Another big problem is that when the material is being cut, the small metal particles released during cutting are apt to form a short circuit between the signal electrode and the earth electrode. When the electrode surfaces are made from silk-screen printed silver pasta, the price becomes very high as silver pasta is very expensive. As compared with the manufacturing method of the invention, the price of an electrode surface printed with silver pasta is multiple times higher.